An understanding of the propensities of specific polypeptides to aggregate is of crucial importance for elucidating the molecular basis of protein deposition diseases, such as Alzheimer's and other amyloid diseases, and for understanding the mechanisms of action of the mutations associated with hereditary forms of such diseases.
In each of the various pathological conditions associated with protein and peptide deposition, a specific peptide or protein that is normally soluble is deposited, either intact or in fragmented form, into insoluble aggregates that accumulate in one or more type(s) of tissue. Numerous mutations have been found to be associated with familial forms of protein deposition diseases and more than 100 have been shown directly to involve the sequence of the peptide or protein responsible for aggregation (Siepen and Westhead, 2002). Many of these mutations have been identified over the past 5 years, and the number is expected to increase dramatically in the near future. Investigation of the mechanisms by which natural mutations result in pathological behaviour has proved to be of fundamental importance for exploring the molecular basis of the underlying disease, even in those cases where they are sporadic rather than familial in origin (Selkoe, 2001; Volles & Lansbury, 2002).
The ability to form highly organised aggregates having common structural characteristics, such as amyloid, has been found to be a generic property of polypeptides, regardless of sequence or structural similarity, and not simply a feature of small numbers of proteins associated with recognised pathological conditions (Dobson, 2001).
In the native state, hydrophobic residues are usually embedded within the core of a protein, thus the opportunity for these residues to interact is limited. However, proteins are dynamic and an equilibrium exists between the stable and folded conformation, and destabilised, partially or fully unfolded states. The free energy value (ΔG, kJ mol−1) for a protein provides an indication of the stability of the protein. Aggregation occurs when proteins in their native state denature; as the protein unfolds, intramolecular bonds are broken, allowing the polypeptide main chain (backbone) and hydrophobic side chains to be exposed. Hydrogen bonds and other interactions can then form between the partially or fully denatured protein molecules, resulting in intermolecular associations and aggregate formation.
In some instances, it may be desirable to form aggregates, in particular fibrils, for example for use as plastic materials, in electronics, as conductors, for catalysis or as a slow release form of the polypeptide, or where polypeptide fibrils are to be spun into a polypeptide “yarn” for various applications; for example, as described in published patent applications WO0017328 (Dobson) and WO0242321 (Dobson & McPhee).
However, in other circumstances the formation of aggregates is disadvantageous, for example, when it is desired to use a polypeptide at concentrations or under conditions desirable for physiological activity, therapeutic administration or industrial application. In particular, the use of bioactive peptides and proteins as pharmaceutical agents is limited where the peptide or protein tends to form aggregates during manufacture, processing, storage or following administration. These issues are widely recognised in the biotechnological and pharmaceutical industry and constitute a major problem and economic burden, that can be difficult to overcome and may require the use of sophisticated expression and refolding techniques, the development of specific formulations, stabilising agents and excipients, cold chain delivery, or immediate reconstitution before use. Almost all known polypeptide therapeutic products present these problems, e.g. insulin, interferon-γ, BMPs, calcitonin, glucagon, antibodies.
Various factors are known to affect the tendency of a polypeptide to aggregate. Some of these factors are local to amino acid residues, other factors are global and can affect the entire protein. For example, when mutations are made in a polypeptide, local factors in the region of the mutation such as increased hydrophobicity, or tendency to convert from α-helix to β-sheet conformation, result in a higher rate of aggregation than that of the wild type (non-mutant) protein. “Global” or overall changes due to mutations can also affect the rate of aggregation; for example, a change in net charge of the mutant polypeptide bringing it closer to neutral results in an increased tendency of a polypeptide to aggregate. Mutations that destabilise the native state of the polypeptide also result in facilitated aggregation.
A detailed mutational study on a model protein, muscle acylphosphatase (AcP), demonstrated that the rate of aggregation for an ensemble of partially denatured conformations can be followed readily for AcP using a variety of spectroscopic probes. The rate of aggregation was determined for over 50 mutational variants of this protein (Chiti et al., 2002a; 2002b: Chiti, F., Taddei, N., Baroni, F., Capanni, C., Stefani, M., Ramponi, G. & Dobson, C. M. Kinetic partitioning of protein folding and aggregation. Nature Struct. Biol. 9, 137-143 (2002a); Chiti, F., Calamai, M., Taddei, N., Stefani, M. Ramponi, G. & Dobson, C. M. Studies of the aggregation of mutant proteins in vitro provide insights into the genetics of amyloid diseases. Proc. Natl. Acad. Sci. USA, 99: 16419-16426 (2002b)). Many of these mutations, particularly those involving residues 16-31 and 87-98, were found to perturb the aggregation rate of AcP very significantly (Chiti et al., 2002a; 2002b). Chiti (2002a) concluded that the measured changes in aggregation rate upon mutation positively correlated with changes in the hydrophobicity and β-sheet propensity of the regions of the protein in which the mutations are located. Chiti (2002b) examined AcP mutations that altered the charge state of the AcP protein without affecting significantly the hydrophobicity or secondary structure propensitities of the polypeptide chain. An inverse correlation was reported between the rate of aggregation of protein variants under denaturing conditions and the overall net charge of the protein.
The factors that affect the rate of aggregation of a protein are diverse. When amino acid substitutions are made in a protein, several factors are involved to different extents. A single mutation can increase the net charge, thereby disfavouring aggregation (for example, the replacement of Ala for Asp in a positively charged protein). Nevertheless, the same mutation can increase hydrophobicity, thereby bringing an accelerating contribution to the aggregation rate. Finally, the same mutation also changes the α-helical and β-sheet propensities of the polypeptide chain, introducing other factors. The relationship between these factors and their relative importance to aggregation (solubility) are not well characterised.
Thus, it has not been possible to predict accurately the tendency of a protein to form insoluble and ordered aggregates, such as amyloid fibrils, nor to predict or calculate the effect of specific amino acid modifications, such as replacements, on aggregation/solubility. The inability to make such predictions or calculations constitutes a problem in the design and/or handling of polypeptides, whether in vivo or in vitro.
The ability to predict polypeptide aggregation is of crucial importance in elucidating the pathogenic effect of the large numbers of mutations associated with protein deposition diseases. Establishment of general principles in aggregation would make it possible to use statistical methods to analyse the relationships between mutation, aggregation and disease. An understanding of the propensities of specific proteins to aggregate would allow the establishment of criteria to modify rationally the aggregational properties of natural or designed peptides and proteins for industrial processes, research purposes, medical treatment or biotechnological application. Furthermore, methods of the invention may be used to identify or design polypeptide sequences with a reduced aggregation propensity, re-designed polypeptides could be administered by methods such as gene therapy to treat certain disorders, particularly those associated with protein aggregation. The ability to identify or design polypeptides with specific aggregation properties will be important for development and manufacture of polypeptides for applications in the material and device areas, such as those described in WO0017328 (Dobson) and WO0242321 (Dobson & McPhee).
It would therefore be useful to be able to determine which parts of an amino acid sequence promote aggregation, to be able to determine whether a particular polypeptide is likely to form insoluble aggregates, and to be able to predict the effect that a particular modification or modifications of amino acid sequence will have on the aggregation/solubility properties of a polypeptide.